Hot runner nozzles allow the melt of injectable plastics to be fed in narrow temperature ranges to the cavities of an injection moulding tool. It is important to make sure that when opening the tool, that is, during the parting of the mould halves after solidification of the moulded part, the still liquid plastic melt can escape from the hot runner nozzle at the supply end. To that end, the hot runner nozzle is closed by a needle. For this purpose, in the nozzle opening or the discharge opening in the direction of flow of the melt the needle is inserted from the back in the discharge opening.
Most of the known hot runner nozzles have needles which are arranged axially displaceable in the centre of the melt runner. This means that the melt is always guided through a particular runner length parallel to the nozzle needle. In order to keep the position of the nozzle needle in the region of the nozzle opening, which usually has a diameter of only approximately 1.2 mm, the needle has a diameter of 4 or 5 mm in the rear region. Only the front end of the needle is accordingly provided with a smaller diameter, so that it can enter the nozzle opening. Such nozzle needles have the disadvantage that on the one hand they necessarily have a large diameter and on the other hand have a large mass as a result, which must be moved in the shortest possible time when closing and opening. Furthermore, additional strong forces from the counter-pressure of the melt result from the large projection surface at pressures in the melt of up to 2500 bar, which must be overcome when closing. High capping forces require correspondingly powerful and consequently large drives in terms of volume for the nozzle needles. Moreover, there is a negative effect on the swirling of the melt along the nozzle needle. It is expensive to accommodate these drives in an injection mould with a plurality of cavities, and the cavities cannot be arranged as close together as would be desirable.
A further hot runner nozzle with a needle valve is known from DE 40 21 782 A1, in which the needle is guided at an acute angle and in a separate tube to the side of the melt runner. The tube with the needle joins the melt runner at a small distance to the nozzle opening in such a way that at the end of the injection process the needle at an acute angle from the back can be pushed through a conical inlet region in front of the cylindrical nozzle bore and stops the melt flow. In fact, no undesirable swirling of the plastic melt occurs in this device during the injection process, since it no longer has to be guided along the needle in the melt runner, but the front end of the needle can close the nozzle opening only along a contact line, but does not enter the cylindrical nozzle runner. This has the effect that the cross-section of the nozzle needle must be significantly larger than the nozzle opening, in order to close the nozzle opening securely on the one hand and on the other hand to resist the lateral, radially acting pressure of the melt upon the needle. A further disadvantage of this device is that due to the large cross-section of the needle in comparison to the cross-section of the nozzle opening, the feed force, which is necessary to advance the needle in a fraction of a second, is relatively large in order to overcome the pressure of up to 2500 bar acting on the end face of the nozzle needle and therefore also the necessarily powerful drive which takes up a lot of space at an acute angle to the side of the nozzle housing.